Spider silk fibers are fibers having strength and stretchability, and are known to have higher toughness than high-tensile steels, Nylon 6 (trademark) fibers, and the like. In addition, they have an advantage in that oil is not used as a raw material and biomass can be used instead. Some artificial spider silk fibers also have been proposed. For example, Patent Document 1 describes a fiber obtained by using, as a spinning solution, a solution in which a synthetic protein derived from natural spider silk proteins is dissolved in hexafluoroisopropanol (HFIP), and subjecting the solution to wet spinning.